In the latest example of the site being used as a conduit to try to influence Hollywood casting decisions, more than 17,000 people have joined a call for producers to remove the duo and replace them with Matt Bomer and Alexis Bledel.

Author EL James, whose steamy novel has sold more than 70m copies worldwide, revealed on Monday that Hunnam and Johnson had been cast as kinky billionaire Christian Grey and blushing virgin Anastasia Steele. The announcement, made on Twitter, ended more than 18 months of furious speculation, but left some fans unhappy.

Bomer, best known for supporting roles in Magic Mike and In Time, as well as the TV show White Collar, has topped a number of fan polls, with Bledel, of Gilmore Girls and Sin City fame, his preferred partner. "Matt Bomer is the PERFECT DESCRIPTION OF CHRISTIAN GREY AND ALEXIS BLEDEL IS THE PERFECT ACTRESS TO REPRESENT ANASTASIA STEELE and if THEY ARE NOT, NOBODY WILL BE And I read the whole trilogy and I can assure that Matt is the perfect actor for this movie and Alexis too," reads the Change.org petition. "So please PLEASE, all of the GREYsessed and Bomerettes in the world NEED those actors. They always will be for us the Christian Grey and Anastasia Steele. WE CAN DO ANYTHING GUYS, ANYTHING."

Fifty Shades of Grey producer Dana Brunetti tried to assuage fan anger. "There is a lot that goes into casting that isn't just looks," he tweeted. "Talent, availability, their desire to do it, chemistry with other actor, etc. So if your favourite wasn't cast, then it is most likely due to something on that list. Keep that in mind while hating and keep perspective."

The novelist Bret Easton Ellis, who last year mounted a high-profile but ultimately unsuccessful campaign to have himself installed as screenwriter for the project, revealed that neither Bomer nor another fan favourite, Ian Somerhalder, were ever in the running for the role of Christian. "EL James and I were at Rob Pattinson's house when she admitted that Rob was first choice for Christian. Ian Somerhalder never in running," he tweeted. "Matt Bomer was [also] never in the running."

Fifty Shades centres on Christian's recruitment of a young virgin, Anastasia, to be his well-remunerated sex slave. It is expected to be the first of a trilogy based on James' novels. Johnson is best known for small roles in The Social Network and The Five-Year Engagement, while Hunnam made his name in the cult US TV show Sons of Anarchy and is Hollywood hot property after positive reviews and good global box office returns for Pacific Rim.

Change.org, originally set up as a platform for ordinary people to influence social change, last month emerged as a place for fans to express fury over casting choices when a petition to replace Ben Affleck as the new Batman picked up more than 90,000 "signatures". The Oscar winner has been cast as the caped crusader in Zack Snyder's forthcoming sequel to this year's Superman reboot, Man of Steel.